For decades, effluent waste gases from various industrial processes, incinerators, sewer beds, rendering plants and the like, have presented serious pollution control problems. This is especially true in recent times when, with a growing worldwide population, urban centers are located in proximity to the sources of production of unwanted, malodorous waste gases. In recognition of the need for removal of unwanted contaminants from waste gas streams, various methods have been developed. In general, the method utilized is tailored to the type of contaminant to be removed.
Frequently, the pollutants are in the form of air borne particles, carried in a waste gas stream emanating from sewer beds and certain industrial processes, such as rendering plants. The particles range in size from relatively large particulates, as seen in flue gases from incinerating operations, to minute particles, sometimes of submicron size, which are developed by rendering plants, sewage treatment plants, and the like.
In the case of the larger flue gas contaminants, wet scrubbers have been utilized for treating the gas and removing the particulates. In a typical wet scrubber, particle removal is achieved when water under pressure is sprayed into a treatment chamber containing the flue gas. The removal phenomenon is generally mechanical, rather than chemical, in nature. Large orifice nozzles, producing large droplets, are suitable, since large aqueous surface areas are not required for mediating chemical reactions. For example, in U.S. Pat. No. 4,305,909, there is disclosed an integrated flue gas processing apparatus including an integrated system utilizing a spray scrubbing tower and a wet electrostatic precipitator.
Other conventional systems, utilizing various gas treatment methods are disclosed in U.S. Pat. Nos. 3,363,403; 3,331,192 and 4,256,468. In general, these systems are highly complex and expensive.
In sharp contrast to such wet scrubbers, such systems are not designed for small particle removal, such as the particle removal requirement for odor control of waste gas treatment. It is not unusual, when such small particles are required to be removed, that conventional odor control systems can be expensive, complex and, in some cases, not very effective.
Wet scrubbing systems are generally not designed for small particle removal, and are designed to operate on hot flue gases. Thus, they are limited in their industrial applications.
In addition to the limitations of the foregoing mentioned conventional odor control systems, there is a problem of unreacted agents being released into the atmosphere. Additionally, in prior known conventional systems, compressed air is used to drive a finely divided water/chemical mixture into a reaction zone, for treating the waste gas. Because compressed air is relatively dry, the water vapor/chemical reagent drops evaporate quickly within the mist scrubbing system, thereby producing a chemical residue. The residue, typically very small particles having a mass median diameter in the order of two microns or less, frequently includes either reactant products or unreacted substances. They may be acidic, or basic, in nature.
Because of the small size of the particles, the residue is generally beyond the removal capability of conventional mist scrubbers. As a result, the chemical residue frequently passes inadvertently into the atmosphere with the treated waste gas.
Thus, while conventional mist scrubbing systems are satisfactory for some applications, the chemical residue passing into the environment has unwanted and undesirable effects on the environment. In addition, in some cases, the residue may present a public health threat to those in proximity to the mist scrubber.
In view of the foregoing, it would be highly desirable to have an odor control apparatus, and method, which would be capable of small particle removal in a safe, efficient and economical manner, and which would reduce substantially the amount of chemical residue released into the atmosphere. Such a system and method should be less complex than conventional odor control systems.
In U.S. Pat. No. 4,125,589 there is disclosed an odor control system for treating gases containing offensive constituents. The gas is passed through a spray treatment zone where the gas is contacted by a finely divided spray, of water and an oxidizing agent. While such a method may remove some odoriferous constituents, it presents several significant drawbacks. For example, because of an emphasis on removing odor from the gas, no practical limits are placed on the volume of oxidizing agent utilized in the process. As a result, an excess of such agent is often used with resulting economic waste and otherwise undesirable results.
In addition, the unreacted treating agents may represent a substantial health hazard when they are released into the atmosphere. For example, it is recognized that some agents, such as bleaches, containing sodium hypochlorite, can be potential hazards if released in unreacted form into the atmosphere. In the case of sodium hypochlorite, the danger of hydrogen chloride formation, after discharge from the stack, is a serious concern.
Often, a surplusage of unreacted reagents released to the atmosphere form from a visible plume exiting the smokestack. In such a situation, the gas leaving the stack may be odor free. However, the plume may contain substantial amounts of unreacted noxious particles being released to the atmosphere. Thus, for example, while the U.S. Pat. No. 4,125,589 discloses a method for removing odoriferous constituents from a waste gas stream, the patented technique does not address the unreacted reagent problem.
In some prior known systems, levels of bleach are reduced, in a manual operation, when a plume is visually observed by an attendant. Such an attempt to control the release of pollutants is satisfactory for many applications. However, in some situations, such as during night operations, observation of the plume may be difficult if not impossible. The net result is that the gas being treated may no longer be odoriferous, but the atmosphere can become polluted to an unacceptable level in such circumstances. In view of the foregoing, it would be highly desirable to have an apparatus and method capable of removing unwanted contaminants from a gas stream, while significantly controlling the amount of unreacted treating agents released into the atmosphere.
With regard to another conventional process, U.S. Pat. No. 4,994,245 discloses a method for removing odors from process air streams by using sulfuric acid and surfactant materials introduced into an air stream. In addition, bleach is utilized for odor removal, and attempts are made to remove odoriferous constituents and noxious particles from the resulting plume. In spite of these efforts, even after the treatment process is completed, unwanted constituents still remain in the gas stream.
In an attempt to deal with such unwanted constituents, the process disclosed by U.S. Pat. No. 4,994,245 requires a dilution fan at the stack to dilute the air stream, thereby making the plume less visually discernable. Of course, the problem of release of unreacted constituents into the atmosphere remains, because the absolute amount of particle discharge is the same. Thus, only a superficial attempt to conceal the existence of the plume has been accomplished, and unreacted substances such as sodium hypochlorite, are continuously inadvertently discharged into the atmosphere during operation of the process.
In addition to the above limitations with regard to the inability to remove sufficient quantities of unwanted constituents from the plume, the last mentioned patent discloses a process requiring three stages of treatment and recirculation of reagents. Thus, a large facility, requiring expensive treatment chambers, plumbing, pumps and reagent containers are all necessary to implement the patented system. As a result, the process is unduly complex and expensive to manufacture. Further, such a process is dependent upon the proper functioning of all components at all times. If a single component of one of the three stages fails, the entire process becomes inoperative.
Thus, it would be desirable to have an odor control system having a much more simplified design. Such a system should be less dependent on proper simultaneous, multistage functioning of complex components.
Another significant drawback of the process disclosed in the last mentioned patent, is the requirement for use of sulfuric acid. The use of such a strong acid can constitute a potential hazard, due to its highly corrosive and caustic characteristics. Thus, utilization of such an acid requires expensive safety precautions and highly trained personnel, thereby adding significantly to system operating costs.
In summary, it will be noted that some conventional systems may efficiently remove odor from waste gas streams, but unreacted noxious particles can be left untreated by the process, only to form an unwanted, noxious plume, which passes into the atmosphere. While some attempts may have been made to dilute the undesirable plume, thereby covering up the release of unwanted particles, sufficient particle removal is not accomplished for many applications. In addition, conventional odor control systems are not entirely effective for monitoring plume constituents so that potentially hazardous constituents, such as unreacted sodium hypochlorite, can pass freely into the atmosphere.
In view of the foregoing, it would be highly desirable to have a new and improved odor control system, which could remove odoriferous constituents from waste gas streams, at the same time greatly reducing the amount of unreacted constituents being released into the atmosphere. Such a system should operate in a less expensive, less complex and safer manner than conventional odor control systems.